Synthesis and Microwave Dielectric Properties of SrLa4−x Pr x Ti5O17 (0 ≤ x ≤ 4) Ceramics

Single-phase ceramics in the SrLa_4?x Pr _x La₄Ti₅O₁₇ (0 ≤ x ≤ 4) series were processed via a solid-state sintering route. X-ray diffraction analysis revealed single-phase ceramics for all the compositions. The molar volume (V _m) decreased while the theoretical density (ρ _th) increased with increase in the Pr content. Substitution of Pr³⁺ decreased the relative permittivity (ε _r) and temperature coefficient of resonant frequency (τ _f) due to its smaller ionic polarizability (α _d) and ionic radius than La³⁺. In the present study, ε _r ≈ 54.2, Q _u f ₀  ≈ 7935 GHz, and τ _f  ≈ ?20.3 ppm/°C were achieved for the composition with x = 2 (i.e., SrLa₂Pr₂Ti₅O₁₇).     

Preparation, Characterization, and Microwave Dielectric Properties of Sr2La3Nb1−x Ta x Ti4O17 (0 ≤ x ≤ 1) Ceramics

Sr₂La₃Nb_1?x Ta _x Ti₄O₁₇ (0 ≤ x ≤ 1) ceramics were processed via a solid-state mixed oxide route. Sr₂La₃Nb_1?x Ta _x Ti₄O₁₇ (0 ≤ x ≤ 1) solid solutions were single phase in the whole range of x values within the x-ray diffraction (XRD) detection limit. The microstructure comprised elongated and needle-shaped grains. The ceramics exhibit relative permittivity (ε _r) of 73 to 68.6, product of unloaded quality factor and resonant frequency (Q _u f ₀) of 7100 GHz to 9500 GHz, and temperature coefficient of resonant frequency (τ _f) of 78.6 ppm/°C to 56.6 ppm/°C.     

Phase, Microstructure, and Microwave Dielectric Properties of NaCa4−x Sr x Nb5O17 (x = 0 to 4) Ceramics

A series of A₅B₅O₁₇-type NaCa_4?x Sr _x Nb₅O₁₇ (x = 0 to 4) compounds were processed through a solid-state mixed-oxide route. All the compositions formed dense single-phase ceramics within the detection limit of an in-house x-ray diffraction facility when sintered at 1300°C. The substitution of Sr for Ca changed the crystal symmetry from monoclinic (x = 0) to orthorhombic (x = 1 to 4) along with a slight increase in molar cell volume due to the relatively larger ionic radius of Sr. The relative permittivity (ε _r) and temperature coefficient of resonance frequency (TCF) increased from 46 to 84 and from ?117 ppm/°C to +377 ppm/°C, respectively, while the quality factor (Q × f) decreased from 11,063 GHz to 559 GHz with an increase in x from 0 to 4. Optimum properties were achieved for NaCa₃SrNb₅O₁₇, which exhibited ε _r = 57, Q × f = 4628 GHz, and TCF = ?41 ppm/°C. Compounds in the NaCa_4?x Sr _x Nb₅O₁₇ series exhibited high ε _r and Q × f with adjustable TCF; however, further work is required for simultaneous optimization of all three properties.     

Thermoelectric Properties of Light-Element-Containing Zintl Compounds CaZn2−x Cu x P2 and CaMnZn1−x Cu x P2 (x = 0.0–0.2)

Light-element-containing CaAl₂Si₂-type Zintl phases CaZn_2?x Cu _x P₂ and CaMnZn_1?x Cu _x P₂ (x = 0.0–0.2) have been synthesized by solid-state reaction. Electrical resistivity (ρ), Seebeck coefficient (α), and thermal conductivity (κ) were measured over a wide temperature (T) range (80–1000 K) to evaluate the thermoelectric potential of these materials. Below 300 K, the power factor (PF; α ²/ρ) is very small. Above 600 K, however, PF increases rapidly for all compositions because of a rapid increase of α and a simultaneous decrease of ρ. The measured large α is consistent with the wider band gap expected for these compositions. Compared with the pure compounds, larger PF values are observed for the Cu-substituted compounds; the largest observed PF is ～0.5 mW/m K². The thermal conductivity is found to be rather low, despite the presence of light elements, and is in the range 1.0–1.5 W/m K at 1000 K. Because of the combination of low κ and moderate PF values, the dimensionless figure of merit ZT = α ² T/ρκ reaches a maximum of 0.4 for CaZn_1.9Cu_0.1P₂.     

Effects of Annealing Temperature on the Electric Properties of 0.94(Na0.5Bi0.5)TiO3–0.06BaTiO3 Ferroelectric Thin Film

0.94(Na_0.5Bi_0.5)TiO₃–0.06BaTiO₃ (NBT–BT6) ferroelectric thin films have been fabricated on Pt–Ti–SiO₂–Si(100) substrate by metal–organic decomposition. The effects of annealing temperature (650–800°C) on the microstructure, and the piezoelectric, ferroelectric, and dielectric properties of the thin films were studied in detail. The residual stress was evaluated by the orientation average method to clarify its dependence on annealing temperature and grain size, and it was correlated with the electric properties to understand the mechanism of piezoelectric enhancement. Among the thin films, NBT–BT6 thin film annealed at 750°C has the largest effective piezoelectric coefficient, 95.1 pm/V, remnant polarization, 49.7 μC/cm², spontaneous polarization, 105.2 μC/cm², and dielectric constant, 504, and the lowest dielectric loss, 0.05, and tensile residual stress, 24.5 MPa. For the NBT–BT6 thin film annealed at 750°C, a wide temperature range, 183–210°C, around the phase transition temperature (T _m) was observed in the dielectric temperature plots, and the diffusion coefficients (γ) were quantitatively assessed as 1.6, 1.78, and 1.6. Piezoelectric performance is discussed on the basis of the dispersion phase transition and residual stress.     

Effect of Excess Na on the Morphology and Thermoelectric Properties of Na x Pb1−x Te0.85Se0.15

The thermoelectric properties of p-Na _x Pb_1?x Te_0.85Se_0.15, which possesses a high thermoelectric figure of merit due to band convergence, have been systematically investigated for increasing Na concentration (x = 0.01, 0.02, 0.03, 0.05, and 0.07) from room temperature to 773 K. For x values up to 0.03, the hole concentration increases with the Na concentration; however, for x ≥ 0.03, excess Na forms separate microstructures with needle- and plate-like shapes. At high concentrations (x = 0.05 and 0.07) both the number and size of these structures increase (over 10 μm). Differential scanning calorimetry identifies a phase change near 660 K in samples with x = 0.05 and 0.07, confirming the formation of microstructures; this phase change leads to a decrease in electrical resistivity. However, these microstructures do not significantly affect thermal transport, probably because they are too large to scatter phonons. The highest thermoelectric figure of merit, zT, value is 1.6, which is obtained at 760 K for x = 0.05, due to the low thermal conductivity and electrical resistivity.     

Thermoelectric Properties of Ca3−x Dy x Co4O9+δ with x = 0.00, 0.02, 0.05, and 0.10

Polycrystalline samples of Ca_3?x Dy _x Co₄O_9+δ (x = 0.00, 0.02, 0.05, and 0.10) have been prepared by conventional solid-state synthesis. The x-ray diffraction (XRD) results revealed that all the samples are single phase. The thermoelectric properties were measured at 25 K to 300 K. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The electrical resistivity of all the samples exhibited the nonmetal-to-metal transition at below 75 K. The electrical resistivity decreased and the thermopower increased with increasing Dy³⁺ content. Among all the samples, Ca_2.9Dy_0.10Co₄O_9+δ had the highest dimensionless figure of merit of 0.044 at 300 K.     

Thermoelectric Properties of (Bi1−x Sb x )2S3 with Orthorhombic Structure

Thermoelectric properties of the substitution system (Bi_1?x Sb _x )₂S₃ have been investigated, where binary Bi₂S₃ and Sb₂S₃ are narrow-gap semiconductors. It is confirmed that metallic conduction, originating from mobile electrons due to production of sulfur vacancies, is observed in Bi₂S₃ over a wide temperature range below room temperature. In Sb₂S₃, mobile carriers are not created and insulating behavior is observed because of the considerably wide bandgap. Change of the carrier number by substitution of antimony contributes strongly to the thermoelectric properties (resistivity and Seebeck coefficient). As a result, the nondimensional figure of merit, ZT, decreases monotonically with increasing antimony content. The maximum value of ZT is obtained in Bi₂S₃ as ZT ≈ 0.1 at room temperature. It is pointed out that control of the carrier number, which is achieved by production of sulfur vacancies, is important to achieve high thermoelectric performance in the (Bi_1?x Sb _x )₂S₃ system. It is possible that the thermoelectric efficiency could be improved by control of the carrier concentration in the bismuth-rich region, including pure binary Bi₂S₃.     

Thermoelectric Properties of Yb1−x (Er,Lu) x Al3 Solid Solutions

Ytterbium trialuminide (YbAl₃) has one of the largest thermoelectric power factors of known materials below room temperature, making it a material of interest for low-temperature thermoelectric devices. However, the high thermal conductivity, which is due to a combination of a large electronic thermal conductivity and a moderately large lattice thermal conductivity, is detrimental to the figure of merit. Substitution of different atoms on the Yb site was performed in order to assess their ability to favorably alter the electronic structure and/or reduce the lattice thermal conductivity. We have synthesized and studied the thermoelectric properties of the solid solutions of YbAl₃ with ErAl₃ and LuAl₃. Results for electrical conductivity, thermal conductivity, and Seebeck coefficient for several of these solid solutions over the temperature range of 80 K to 300 K are reported. Although most substituted samples are driven toward a metallic state, we find that for some compositions the figure of merit is enhanced relative to pure YbAl₃.     

Effects of thermal annealing on the morphology of the AlxGa(1−x)N films

Effects of thermal annealing on the morphology of the Al_xGa_(1−x)N films with two different high Al-contents (x=0.43 and 0.52) have been investigated by atomic force microscopy (AFM). The annealing treatments were performed in a nitrogen (N₂) gas ambient as short-time (4 min) and long-time (30 min). Firstly, the films were annealed as short-time in the range of 800–950 °C in steps of 50–100 °C. The surface root-mean-square (rms) roughness of the films reduced with increasing temperature at short-time annealing (up to 900 °C), while their surface morphologies were not changed. At the same time, the degradation appeared on the surface of the film with lower Al-content after 950 °C. Secondly, the Al_0.43Ga_0.57N film was annealed as long-time in the range of 1000–1200 °C in steps of 50 °C. The surface morphology and rms roughness of the film with increasing temperature up to 1150 °C did not significantly change. Above those temperatures, the surface morphology changed from step-flow to grain-like and the rms roughness significantly increased.